Gold nanostructures by directional solid-state decomposition

Abstract

The trend in electronics industry towards miniaturisation is leading to increasing research focus on nano-electronic devices. For example, metal nanowires show potential for future application as connector lines in such devices. The controlled fabrication of these structures at scales beyond the current limits of lithographic techniques is of great interest. Self-assembly processes using gold nanoparticles are being extensively studied as one possible long-term solution. This study addresses a novel approach of obtaining gold nanostructures. It consists of applying directional solidification processing to eutectoid alloys in order to produce ordered structures, followed by phase separation by selective etching.The directional transformation of eutectoid alloys is a close derivation of the directional solidification of eutectics. In this case, two phases precipitate simultaneously from a high-temperature solid phase (Figure 1). When the growth properties of the two phases are sufficiently compatible, unidirectional decomposition can also produce aligned composite materials [1,2]. One of the most prominent examples of eutectoid reaction is the formation of pearlite by undercooling a Fe-C alloy in the ␥-state. Microstructures are best when the high-temperature phase is nearly singlecrystalline. Therefore, aligned eutectoids are often produced by accomplishing unidirectional solidification and unidirectional decomposition sequentially. Eutectic solidification and eutectoid decomposition, as examples of duplex crystal growth, are inherently more complex than single-phase crystal growth. The technique used in both cases is quite similar: a specimen is unidirectionally translated through a steep temperature gradient and traversed by a macroscopically planar transformation front. If adequate experimental conditions are arranged, the eutectoid reaction product is an aligned microstructure. The conditions to be met are: i) sufficiently steep gradient to avoid both nucleation before the actual position of the transformation front and coalescence afterwards, ii) a translation rate which permits the attainment of a transformation front with stationary moving conditions, i.e., as a first approximation, a lower translation rate than the maximum eutectoid growth rate.Although eutectic and eutectoid reactions are essentially the same in nature, there are some differences that must be pointed out. Firstly, eutectoid spacings are much finer than the eutectic spacings for the same growth rate, due to the lower values of diffusion coefficients in the solid state as compared to the liquid. Also, a decrease in eutectoid spacing, , with increasing growth rate, V, is generally slower for eutectoids than for eutectics. Data can again be fitted to a formula n V = const. However, whereas n = 2 fits to almost all data for eutectics, values of n ranging from 2 to 4 have been reported for eutectoids. The value n = 2 is consistent with volume-diffusion control, and n = 3 is predicted for grain-boundary-diffusion control.

Reactivity of Gold Nanobelts with Unique {110} Facets

“…[33][34][35][36] Briefly, pre-alloys were prepared using 99.999 % Au and high-purity Fe, which was zone-refined five times in-house, by induction melting under inert atmosphere and drop-casting into a cylindrical copper mould. Subsequently, the as-cast ingots were inserted into alumina crucibles and processed in a Bridgman-type crystal-growing facility.…”

Preparation and specific properties of single crystalline metallic nanowires

“…The main difference from eutectic is that due to the lower values of diffusion coefficients in the solid state as compared to the liquid, www.pss-b.com eutectoid spacings are much finer than the eutectic spacings for the same growth rate, which enables obtaining finer nanostructures. Indeed, it was shown [59] that a range of gold nanostructures might be achieved depending on the processing route. For instance, the combination of directional solidification and solid-state transformation of a Fe-Au eutectoid followed by a precisely controllable electrochemical treatment enabled the production of arrays with isooriented nanobelts of a desired length.…”

Structural characterisation of gold nanowire arrays

“…Recently, a method developed for making various nanostructures from directionally solidified eutectics [14][15][16] was extended to Fe-Au eutectoid system [17]. It is based on directional solidstate decomposition of eutectoid alloys in order to produce highly ordered structures, followed by phase separation by selective etching.…”